fix: clamp reserveTokensFloor to prevent negative memory flush threshold

When reserveTokensFloor equals contextWindowTokens, the flush threshold
becomes zero and memory flush never triggers. Clamp reserveTokens so the
threshold always remains positive.

Fixes #50611

src/auto-reply/reply/memory-flush.ts

@@ -197,9 +197,12 @@ export function shouldRunMemoryFlush(params: {
     return false;
   }
   const contextWindow = Math.max(1, Math.floor(params.contextWindowTokens));
-  const reserveTokens = Math.max(0, Math.floor(params.reserveTokensFloor));
   const softThreshold = Math.max(0, Math.floor(params.softThresholdTokens));
-  const threshold = Math.max(0, contextWindow - reserveTokens - softThreshold);
+  const reserveTokens = Math.min(
+    Math.max(0, Math.floor(params.reserveTokensFloor)),
+    Math.max(0, contextWindow - softThreshold - 1),
+  );
+  const threshold = contextWindow - reserveTokens - softThreshold;
   if (threshold <= 0) {
     return false;
   }

src/auto-reply/reply/reply-state.test.ts

@@ -357,6 +357,17 @@ describe("shouldRunMemoryFlush", () => {
       }),
     ).toBe(false);
   });
+
+  it("triggers when reserveTokensFloor equals contextWindowTokens", () => {
+    expect(
+      shouldRunMemoryFlush({
+        entry: { totalTokens: 199_000, compactionCount: 0 },
+        contextWindowTokens: 200_000,
+        reserveTokensFloor: 200_000,
+        softThresholdTokens: 4_000,
+      }),
+    ).toBe(true);
+  });
 });
 
 describe("hasAlreadyFlushedForCurrentCompaction", () => {
@@ -483,4 +494,4 @@ describe("incrementCompactionCount", () => {
     // totalTokens unchanged
     expect(stored[sessionKey].totalTokens).toBe(180_000);
   });
-});
+});